Users flagged it. That's basically always the reason when you see [flagged] on a submission. (With comments it's a little more ambiguous - moderators will also sometimes put [flagged] on a comment, though still in a tiny minority of cases.)

https://news.ycombinator.com/newsfaq.html#flag

We sometimes turn off flags, especially when an article seems to contain enough significant new information to support a substantive discussion. I've done that in this case.

Thanks to the mods for unflagging this as of 1419 EDT Sat

Users with enough karma can flag things they don’t like.